Consider the Federal response, in Lincoln's words, to the opening of hostilities started by South Carolina's firing on Ft. Sumpter. The reason given in the proclaimation was the inability to collect duties due the Federal govenment due to secession.
Six days prior to the opening of hostilities Secretary of War Cameron sent the following instructions to Captain V. Fox, commander of the task force formed to relieve the Federal garrison at Ft. Sumter.:

War Department, Washington, April 6th, 1861
Sir- It having been decided to succor Fort Sumter, you have been selected for this important duty. Accordingly, you will take charge of the transports in New York, having the troops and supplies on board, to the port of Charleston harbor, and endeavor, in the first instance, to deliver the subsistence. If you are opposed in this, you are directed to report the fact to the senior naval officer of the harbor, who will be instructed by the Secretary of the Navy to use his entire force to open a passage, when you will, if possible, effect an entrance, and place both the troops and supplies in Fort Sumter.
Iam,sir,etc. Simon Cameron, Sec. of War

War it was. The importance of the slavery issue, always the key point of the conflict to the Abolishonist faction,only took on paramount importance as the Federal military began to run into manpower shortages and enquiries from front line commanders began to pile up requesting advice as to what to do with the former slaves that were within the areas of their command in the South.

Fully one quarter of the population of the South were for a maintainance of the Union.

One can debate the right or wrong of the causes forever. A writer name of F.C. Bradlee penned the following words regarding the contest.: "It is inconceivable that intelligent men charged with the conduct of public affairs would have plunged their States, unprepared, into so unequal a war. However well assured they may have been of the right of a State to withdraw from the Union, or however strong may have been their convictions that seperation from the Northern States would contribute greatly to the prosperity and happiness of their own States, they would surely have deferred the practical assertion of the right of secession until some adequate preparation had been made for the maintainance of their independence.The South had no navy and almost no means of building one up of sufficient strength in time to be of any avail in the defence of hundreds of miles of seacoast- a seacoast which was practically undefended, and, therefore, at the mercy of a hostile naval power. Thus the capture, in November, 1861, of Port Royal, South Carolina, by the land and naval forces of the United States, commanded respectively by General Burnside and Flag Officer Du Pont, was a most serious blow to the Southern Confederacy."